Multi-beam communications satellites (e.g., spot beam satellites) are designed such that a given geographic coverage area is serviced by a pattern of beams. With such multi-beam satellites, in order to avoid or minimize inter-beam interference, certain frequency reuse principles must be applied to the bream patterns of the antenna design. One of the primary guidelines for the beam pattern is that a frequency and polarization combination of one beam cannot be “reused” within a certain distance from another beam of the same frequency and polarization combination. The reuse distance between beams is generally specified as the distance between beam centers of two beams of a same color (two beams of the same frequency—the same frequency band and polarization), where the distance is quantified in terms of the center-to-vertex radius r of the circumscribing hexagon representing the beams. If the minimum distance requirements are not followed with regard to two such beams, then the beams will cause unacceptable levels of interference between them. The beam pattern design is commonly referred to as a frequency reuse pattern, where each polarization/frequency pair is diagrammatically reflected by a beam color (or pattern in the case of the black and white figures included herein). In typical systems, a reuse of four means that a set of four adjacent beams will have disjoint frequency and polarization assignments such that none of the beams of each set interfere with each other. In other words, adjacent sets of four beams separate the beams sharing a common frequency and polarization such that (even though they are reusing the same frequency and polarization assignments) the beams of one set will not interfere unacceptably with the respective beams of an adjacent set.
For example, FIG. 1A illustrates an example four-beam reuse pattern of a single satellite 110, where, for example, the striped pattern 101 reflects a right-hand polarization of a first frequency or frequency band, the dot pattern 103 reflects a left-hand polarization of the same frequency band as that of 101, the checkered pattern 105 reflects a right-hand polarization of a second frequency or frequency band, and the brick pattern 107 reflects a left-hand polarization of the same frequency band as that of 105. In such a four-color reuse pattern, the distance between the beams or beam centers of two beams of the same color are 2√{square root over (3)}*r apart. As a further example, FIG. 1B illustrates an example three-beam reuse pattern, where (similar to the four-beam reuse pattern of FIG. 1A) each of the patterns 111, 113, 115 reflects a particular beam frequency/polarization assignment. In such a three-color reuse pattern, the distance between the beams or beam centers of two beams of the same color are 3*r apart. Accordingly, as illustrated by these Figures, each group of four or three particular polarization/frequency beams is geographically arranged such that a beam of a particular polarization/frequency is not adjacent to any beam of the same polarization/frequency (where such beam pairs of a same polarization/frequency are separated by a required minimum distance).
FIG. 1C illustrates example frequency band and polarization assignments for the beams of the four-beam reuse pattern of FIG. 1A. Each Beam A, for example, comprises signal A in the frequency band 18.3-18.8 GHz (500 MHz of spectrum for each such beam), applied to an RHCP feed of the downlink antenna. Each Beam B comprises signal B in the frequency band 19.7-20.2 GHz (500 MHz of spectrum for each such beam) applied to an RHCP feed of the downlink antenna. Each beam C comprises signal C in the frequency band 18.3-18.8 GHz (500 MHz of spectrum for each such beam) applied to an LHCP feed of the downlink antenna, and each beam D comprises signal D in the frequency band 19.7-20.2 GHz (500 MHz of spectrum for each such beam) applied to an LHCP feed of the downlink antenna.
FIG. 1D illustrates a block diagram of a configuration for two transmitters configured to transmit one set of the A, B, C, D (or 1, 2, 3, 4) signals to the satellite downlink antenna beams of the four-color reuse pattern of FIG. 1A. With reference to FIG. 1A, each of the beams of the four-color reuse pattern corresponds to a respective one of the RF signals A, B, C, D (as transmitted by a respective transmitter of FIG. 1D). Each of these four RF signals is transmitted by a feed on the downlink satellite antenna to form Beams A, B, C, D. Each of the transmitters comprises an amplifier 131, 151 (e.g., a traveling wave tube amplifier (TWTA)) and a filter 133, 153, and a circular polarizing feed 135, 155. For example, the A+B RF signals (e.g., 1000 MHz) are amplified via the TWTA 131 and the C+D RF signals (e.g., 1000 MHz) are amplified by the TWTA 151. The amplified A+B and C+D signals are then fed into the filters 133, 153, respectively, which separate the combined A+B and C+D into separate A, B, C, D RF signals. Each filter output is connected to a circular polarizing feed 135, 155, whereby the amplified A, B, C, D signals form two circularly polarized beams (e.g., of 500 MHz each). For example, with reference to FIG. 1D, a right-hand polarized Beam A (e.g., 500 MHz) and a right-hand polarized Beam B (e.g., 500 MHz) via the filter/polarizer 133/135, and a left-hand polarized Beam C (e.g., 500 MHz) and a left-hand polarized Beam D (e.g., 500 MHz) via the filter/polarizer 153/155.
FIG. 1E illustrates example frequency band and polarization assignments for the satellite downlink antenna beams of the three-beam reuse pattern of FIG. 1B. Each Beam A comprises the RHCP signal for the frequency band 18.2-19.2 GHz (1000 MHz of spectrum), each Beam B comprises the RHCP signal for the frequency band 18.2-19.2 GHz (1000 MHz of spectrum), and each Beam C comprises the LHCP and RHCP signals for the band 20.0-20.5 GHz (500 MHz of spectrum at the each of the two polarizations RHCP and LHCP totaling 1000 MHz of spectrum). Note that, for this configuration, the ground terminals configured to receive the beam C would be required to have good cross-polarization discrimination.
FIG. 1F illustrates a block diagram of a configuration for two transmitters configured to transmit one set of the A, B, C (or 1, 2, 3) signals to the satellite downlink antenna beams of the three-color reuse pattern of FIG. 1B. With reference to FIG. 1B, each of the beams of the three-color reuse pattern corresponds to a respective one of the RF signals A, B, C (as transmitted by a respective transmitter of FIG. 1F). Each of these three RF signals is transmitted by a feed on the downlink satellite antenna to form Beams A, B, C. Each of the transmitters comprises an amplifier 171, 191 (e.g., a traveling wave tube amplifier (TWTA)), a filter 173, 193, and a circular polarizing feed 175, 195. For example, the Feed 1 RF signal (e.g., 1500 MHz) is amplified via the TWTA 171 and the Feed 2 RF signal (e.g., 1500 MHz) is amplified by the TWTA 191. The amplified signals are then each fed into the filters 173, 193, respectively, which separate the signals into the respective A, B, C beam RF signals. Each filter output is connected to a circular polarizing feed 175, 195, whereby the amplified signals form the respective circularly polarized beams. For example, with reference to FIG. 1F, a right-hand polarized A beam (e.g., 1000 MHz) and a right-hand polarized partial C beam (e.g., 500 MHz) via the filter/polarizer 173/175, and a left-hand polarized B beam (e.g., 1000 MHz) and a left-hand polarized partial C beam (e.g., 500 MHz) via the filter/polarizer 193/195. The spectrum of the two partial C beams combine to provide a total C beam spectrum of 1000 MHz.
Satellite systems are generally designed to maximize capacity by using all of the available spectrum. For example, if 1000 MHz of spectrum (in both polarizations-right-hand polarization (RHCP) and left-hand polarization (LHCP)) is available for a particular system, the system theoretically has 2000 MHz of available spectrum for each beam group. With reference to the four-pattern reuse system of FIG. 1A, for example, each beam represented by the pattern 101 may comprise a RHCP of the frequency band 18.3-18.8 GHz, each beam represented by the pattern 103 may comprise a LHCP of the frequency band 18.3-18.8 GHz, each beam represented by the pattern 105 may comprise a RHCP of the frequency band 19.7-20.2 GHz, and each beam represented by the pattern 107 may comprise a LHCP of the frequency band 19.7-20.2 GHz. Each beam would thereby comprise 500 MHz of spectrum or bandwidth, for a total available capacity of 2,000 MHz within each four-beam group. The reuse pattern can be repeated as many times as desired, up to a maximum desired coverage region, as limited by applicable physical constraints, such as total power and mass limits of the overall satellite payload. The total system bandwidth is then the sum of the individual bandwidths of all the beams.
The Ka frequency band downlink comprises 1500 MHz on each polarization in the United States (U.S.) and as much as 2000 MHz in other regions. In the U.S., the Ka band is the band from 18.3-19.3 GHz and 19.7-20.2 GHz. In other regions, the Ka band may also include the band from 17.8-18.3 GHz. In a satellite system that primarily serves the continental United States (CONUS), the band may be provided as a 3 color reuse plan by frequency division. Specifically, each 1500 MHz may be divided into 6 colors, amounting to 12 colors when factoring in both polarizations. The 12 bands may then be grouped into three sets of four bands each, where the three sets are then routed through three distinct antennas on the satellite. The three-color reuse may also be accomplished by providing all 3000 MHz via a single beam and time hopping the beam over 3 cells, so each cell receives 1000 MHz on average. This approach, however, suffers from the disadvantage of requiring that every feed on each satellite antenna be dual-pol (operate in both polarizations).
The size of a spot beam is determined primarily by the size of the antenna on the satellite—the larger the antenna, the smaller the spot beam. Further, as would be recognized by one of ordinary skill in the art, in order to achieve reasonably acceptable RF performance, the number of beams and the reuse pattern employed will impose certain payload design requirements, such as the number of antennae and the size of each antenna required to implement the desired beam pattern. To cover the eastern half of the continental United States (CONUS), for example, one might design a satellite payload with 50 beams, each of approximately 0.5 degrees diameter, using a three-color reuse pattern. The antennas of such a payload might each be approximately 2.5 m in diameter and three or even four such antennae (e.g., 110a, 110b, 110c, 110d) may be required to achieve desired RF performance. Each beam may be assigned 666 MHz, yielding a total of 33.3 GHz of bandwidth. Accordingly, the desired number of beams, reuse pattern and total capacity will contribute to payload size, weight and power requirements, which in turn will drive up the satellite manufacturing and launch costs.
Moreover, in practice, the distribution of users and associated capacity demand within the coverage area is non-uniform, which drives the goal of a satellite system design to provide a corresponding non-uniform distribution of capacity density to satisfy the respective demand. Accordingly, in recent times, some satellite system designs have attempted to solve capacity density requirements by deploying such satellite technologies as steerable beams. FIG. 1G illustrates the four pattern reuse plan of FIG. 1A, where the beams 1, 2, 3, 4 represent the patterns 101, 103, 105, 107, respectively, and the beam pattern has been overlaid on a map of the Northeastern United States. As further illustrated in FIG. 1G, in order to provide higher capacity density to the New York/Long Island, Southern Connecticut and Boston areas, certain of the beams have been steered to double the capacity over these regions (e.g., the 3 beam 121 has been moved to the cell 122, the 1 beam 123 has been moved to the cell 124, the 3 beam 125 has been moved to the cell 126, and the 2 beam 127 has been moved to the cell 128). Accordingly, the capacity density has been adjusted to double the spectrum/capacity delivered to the cells 122, 124, 126, 128. This capacity density adjustment, however, has been achieved at the expense of the capacity delivered to the cells 121, 123, 125, 127—as spectrum cannot be provided to these cells without violating the adjacent cell polarization/frequency requirements.
An alternative design may provide for a higher per-beam spectrum allocation. In view of such constraints as satellite size, weight and power, however, such a design would limit the total number of beams available at the higher spectrum allocation. Further, providing for such high capacity beams also significantly increases satellite complexity. Accordingly, with this design, there may not be enough user beams to cover the contiguous United States, and thus the capacity would have to be provided to the higher density population areas at the expense of having no capacity provided to the lower density population areas (e.g., providing user beams over only the Eastern and Western coasts of the United States. Accordingly, again, the desired capacity density allocation is achieved at the expense of being unable to provide capacity to certain geographic regions.
What is needed, therefore, are approaches for flexible capacity allocation in satellite communications systems, without sacrificing capacity in adjacent beams and without adverse impact with regard to satellite size, weight, power and complexity constraints.